A number of medical ailments are treated and/or diagnosed through the application of a magnetic field to an afflicted portion of a patient's body. Neurons and muscle cells are a form of biological circuitry that carry electrical signals and respond to electromagnetic stimuli. When an ordinary conductive wire loop is passed through a magnetic field or is in the presence of a changing magnetic field, an electric current is induced in the wire.
The same principle holds true for conductive biological tissue. When a changing magnetic field is applied to a portion of the body, neurons may be depolarized and stimulated. Muscles associated with the stimulated neurons can contract as though the neurons were firing by normal causes.
A nerve cell or neuron can be stimulated in a number of ways, including transcutaneously via transcranial magnetic stimulation (TMS), for example. TMS uses a rapidly changing magnetic field to induce a current on a nerve cell, without having to cut or penetrate the skin. The nerve is said to “fire” when a membrane potential within the nerve rises with respect to its normal negative ambient level of approximately −90 mV, depending on the type of nerve and local pH of the surrounding tissue.
The use of magnetic stimulation is very effective in rehabilitating injured or paralyzed muscle groups. Apart from stimulation of large muscle groups such as the thigh or the abdomen, experimentation has been performed in cardiac stimulation as well. In this context, magnetic stimulation of the heart may prove to be superior to CPR or electrical stimulation, because both of those methods undesirably apply gross stimulation to the entire heart all at once.
Another area in which magnetic stimulation is proving effective is treatment of the spine. The spinal cord is difficult to access directly because vertebrae surround it. Magnetic stimulation may be used to block the transmission of pain via nerves in the back, e.g., those responsible for lower back pain.
Magnetic stimulation also has proven effective in stimulating regions of the brain, which is composed predominantly of neurological tissue. One area of particular interest is the treatment of depression. It is believed that more than 28 million people in the United States alone suffer from some type of neuropsychiatric disorder. These include conditions such as depression, schizophrenia, mania, obsessive-compulsive disorder, panic disorders, and others. Depression is the “common cold” of psychiatric disorders, believed to affect 19 million people in the United States and possibly 340 million people worldwide.
Modern medicine offers depression patients a number of treatment options, including several classes of anti-depressant medications (e.g., SSRI's, MAOI's and tricyclics), lithium, and electroconvulsive therapy (ECT). Yet many patients remain without satisfactory relief from the symptoms of depression. To date, ECT remains an effective therapy for resistant depression; however, many patients will not undergo the procedure because of its severe side effects.
Recently, repetitive transcranial magnetic stimulation (rTMS) has been shown to have significant anti-depressant effects for patients that do not respond to the traditional methods. The principle behind rTMS is to apply a subconvulsive stimulation to the prefrontal cortex in a repetitive manner, causing a depolarization of cortical neuron membranes. The membranes are depolarized by the induction of small electric fields in excess of 1 V/cm that are the result of a rapidly changing magnetic field applied non-invasively.
To produce the rapidly changing magnetic field needed to induce a current on a nerve cell in connection with any of the above treatments, a magnetic stimulation device typically includes one or more energy storage capacitors. The capacitors provide the magnetic stimulation device with the high power and quick re-charging time needed to repetitively pulse a magnetic field generator, such as a magnetic core, in a manner required by the desired treatment regimen. Because the capacitors in such an arrangement directly power the magnetic core that provides the magnetic pulses to a patient, any change in the capacitance directly changes the magnetic energy of the pulse delivered to the patient.
Thus, in a magnetic stimulation device that includes a single capacitor, if the single capacitor suffers a complete failure (e.g., open circuit condition), the failure can be easily detected—either because the magnetic core will not pulse, or because the generated pulse will be substantially different from the expected pulse. Conventional start-up diagnostic routines can typically detect this type of failure. Alternately, these failures may be obvious to the treatment provider. For example, readings from a sensor that detects the magnetic field pulse generated by the magnetic core (i.e., a “field sensor”) may be used to determine that the generated magnetic pulse does not have one or more desired characteristics as a result of a capacitor failure.
In some situations, however, the performance of one or more capacitors may degrade slightly over time. For example, one or more capacitors may experience excessive parametric drift. In addition, more than one capacitor may be arranged in a multi-capacitor bank, in which the capacitors are typically connected in parallel. In such a situation, when a single capacitor fails (e.g., causes an open circuit) the overall capacitance of the bank may only decrease by a small percentage. In such situations, the generated pulse may vary from the desired pulse in subtle ways that conventional field sensors cannot detect because the field sensing equipment may have a wide pass/fail threshold to allow for component tolerances and sensor positional variation of the field sensor's position.
As a result, the magnetic device may appear to operate correctly, but actually may be producing magnetic pulses outside of published device specifications, potentially resulting in improper therapy being delivered to the patient. Delivering an incorrect magnetic pulse to a patient can affect the magnetic stimulation treatment adversely. For example, the treatment provider may believe that the patient is not responding to the treatment, when in fact the intended treatment is not being delivered to the patient. Thus, the treatment provider may be lead to make treatment decisions based on faulty information.
In addition, conventional mechanisms for determining whether the generated magnetic pulse is within tolerances have such a coarse level of magnetic field detection that “pass/fail” determinations are the only type of result that can be provided to a treatment provider. Thus, a treatment provider that is confronted with a magnetic stimulator failure does not know how far out of tolerance the magnetic device actually is, and either has to end the treatment or use a spare device, if one is available. Either course of action is usually inconvenient and potentially expensive.